Impregnated, resin bonded mica tape is well known as a high performance insulating tape, particularly useful as ground insulation for high voltage motor or generator coils, as taught by Smith et al., in U.S. Pats. No. 4,020,017 and 4,204,181. The conventional method used to manufacture resin bonded mica tape involves two separate operations and two separate polymeric or resinous solutions. The mica sheet stock used, typically 600 yards in length and 40 inches wide, consists of small mica flakes held together only by molecular attraction and possibly some hydrogen bonding. This material is very weak and must be treated with a mica bond resin. In practice, an epoxy or polyester resin is applied in a solvent solution to the mica flake sheet as the mica bond resin. After the solvent is removed in a conveyor oven, the resin bonds the mica flakes together. At this stage the mica bond resin is dried but not cured. As a result of this procedure the mica sheet can then be further processed.
Althoguh some strength has been added to the mica by this resin treatment, it is far from what is required if it is to be applied to coils by high-speed machine taping heads. To provide the required level of tensile strength and support, and treated mica sheet must be bonded to a glass fabric or other suitable flexible backing material.
In current practice, the mica sheet now containing about 10 weight percent mica bond resin is directed to a second process station in which an adhesive in a solvent solution is applied either to the mica or to the glass fabric. The solvent is removed through a conveyor oven and the tacky adhesive surface is directed to come in contact with the surface to be bonded.
This is a delicate process when the adhesive is applied to the mica, since any solvent in the adhesive system tends to soften the uncured mica bond resin in the mica sheet. If the adhesive is applied to the glass fabric another problem must be addressed. The best glass for the end use of coil insulation is an open weave fabric having large interstices. Such a fabric has ample strength, low cost, and provides easy penetration in the subsequent resin impregnation operation which is applied in the final coil processing. Unfortunately the applied adhesive can strike through this open weave glass, causing a plating-out of the adhesive onto the processing rolls, with the result that clean-up must be performed at regular intervals. For these reasons the glass reinforced mica tape currently available is quite expensive.
In all cases, the resin bonded mica tape must be porous, so that subsequent vacuum pressure impregnation of insulating resin is possible. The resin bonded mica tape will usually contain from about 2 wt. % to about 15 wt. % resin solids, based on the total of mica, flexible backing, and resin weight. What is needed is a simplified, low cost process to make resin bonded mica tape, which will not block upon rolling for storage, and which can be easily impregnated with insulating resin after being wound on an electrical coil.